Contact stress sensor

ABSTRACT

A contact stress sensor includes one or more MEMS fabricated sensor elements, where each sensor element of includes a thin non-recessed portion, a recessed portion and a pressure sensitive element adjacent to the recessed portion. An electric circuit is connected to the pressure sensitive element. The circuit includes a thermal compensator and a pressure signal circuit element configured to provide a signal upon movement of the pressure sensitive element.

This application is a continuation-in-part of U.S. patent applicationSer. No. 11/869,874, titled: “Microelectromechanical Systems ContactStress Sensor,” filed Oct. 10, 2007, incorporated herein by reference,which is a continuation of U.S. patent application Ser. No. 11/143,543,filed Jun. 1, 2005, incorporated herein by reference, which claimspriority to U.S. Provisional Patent Application Ser. No. 60/629,271,filed Nov. 17, 2004, incorporated herein by reference.

The United States Government has rights in this invention pursuant toContract No. DE-AC52-07NA27344 between the United States Department ofEnergy and Lawrence Livermore National Security, LLC.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to microelectromechanical systems (MEMS)processing technology, and more specifically, it relates to embodimentsof contact stress/pressure sensors formed by MEMS processes.

2. Description of Related Art

Many industrial applications and research projects require the stressnormal to contacting surfaces to be known. Several examples include:designing rollers, designing gaskets and seals, robotic tactile sensors,prosthetics, shoes, surgical instruments, automotive crash tests, brakepads and cartilage studies. There are currently three methods fordetermining the stresses between contacting surfaces: analyticcalculation, computational methods and direct measurement (1). Generalcontact problems are very difficult to solve analytically as geometriesand material properties are typically complex and not necessarily known.If these characteristics are known, complex shapes and irregular andchanging contact areas are extremely difficult to model. Additionally,deformation of contacting surfaces introduce non-linearities thattypically make analytic approaches very difficult or impossible tosolve. Computationally, among solid stress/strain modeling efforts,contact problems are considered the most difficult to model. Similar toanalytic methods, modeling of complex and/or compliant surfaces is avery difficult and computationally intensive problem. Very largedeformations and slip between surfaces add further difficulty for FiniteElement Analysis (FEA). Even when it can be used, FEA benefits fromdirect measurement of contact stress for model validation. Due to thedifficulty or impossibility of predicting contact stresses by analyticor computational methods, direct measurement of contact stress is oftenthe only means by which they may be known. Thus, a distinct and verybroad need for measurement instrumentation exists. Despite thewidespread need for contact stress measurement instrumentation, greatprogress remains to be made towards the development of contact stresssensors.

SUMMARY OF THE INVENTION

The invention is a minimal thickness contact stress sensor and includesa variety of embodiments configured as a single sensor or as an array ofsensors. Use is made of a silicon sensing element that has excellentelasticity, long term stability, sensitivity and bandwidth. The devicesenable short and long term applications of either static or dynamicmeasurements.

An embodiment of the invention is a contact stress sensor that includesone or more sensor elements, wherein each sensor element of said sensorelements includes a non-recessed portion no thicker than 280 μm, arecessed portion and a pressure sensitive element adjacent to saidrecessed portion; and an electric circuit operatively connected to saidpressure sensitive element, wherein said circuit includes a thermalcompensator and further includes a pressure signal circuit elementconfigured to provide a signal upon movement of said pressure sensitiveelement. The one or more sensor elements comprises a configurationselected from the group consisting of a single sensor element and aplurality of sensor elements and may further comprise a package withinwhich said sensor element is located, wherein said package comprises athickness no greater than 300 μm. The plurality of sensor elements canbe configured in an array selected from the group consisting of a onedimensional array and a two dimensional array. MEMS processingtechnology is utilized to manufacture the elements of the presentinvention. The MEMS process may comprise anisotropic etching. The sensorelement usually comprises silicon and particularly may comprise a singlecrystal of silicon. The pressure sensitive element can take a variety ofshapes. Exemplary embodiments of the pressure sensitive element aredescribed herein as a diaphragm or a cantilever. The pressure sensitiveelement can be manufactured by a wafer processing technology such as wetcleans, photolithography, ion implantation (wherein at least one dopantis embedded in said sensor element to create a region of increased ordecreased conductivity), dry etching, wet etching, plasma ashing, athermal treatment (selected from the group consisting of a rapid thermalanneal, a furnace anneal and thermal oxidation), chemical vapordeposition, physical vapor deposition, molecular beam epitaxy,electrochemical deposition, chemical-mechanical planarization, wafertesting and wafer backgrinding.

In an embodiment of the present invention the contact stress sensor ispackaged and the package may comprise a base layer, a mid-layer with anopening for said sensor element, and a capping layer, wherein saidmid-layer is between said base layer and said capping layer. Aconductive trace can be located between said base layer and saidmid-layer and is in electrical contact with said electrical circuit. Thebase layer, said mid-layer and said capping layer can comprise polyimideand solder may be located between said conductive trace and saidcircuit. In some embodiments, the base layer comprises polymer, whereinsaid mid-layer and said capping layer each comprise at least oneadhesive layer and at least one polymer layer. The mid-layer may furthercomprise a solder shim layer configured to uniformly support said sensorelement between said sensor element and said conductive trace. Thesolder shim layer is a means for providing a deterministic solderthickness between said sensor element and said conductive trace. In someembodiments, the conductive trace is in movable electrical contact withsaid electric circuit. For example, the conductive trace may not bebonded to said electric circuit. Some embodiments include a means forrelieving residual stress curl of said package.

Exemplary embodiments of the contact stress sensor include a diaphragm,which may or may not be sealed to create a reference volume. The packageor sensor element can include a load amplification element in operativecontact with said pressure sensitive element. This load amplificationelement may be an electroplated bump. The exemplary electric circuitcomprises boron implanted into said pressure sensitive element, whereinsaid boron has been annealed. In some embodiments, the pressuresensitive element comprises a diaphragm, the thermal compensatorcomprises at least one piezoelectric circuit operatively placed on saiddiaphragm, wherein said pressure signal circuit element comprises atleast one piezoelectric circuit operatively placed on said diaphragm. Asingle crystal of silicon is often used for the sensor element andcomprises a <100> crystalline axis and a <110> crystalline axis, whereinsaid thermal compensator is operatively placed on said <100> crystallineaxis and wherein said pressure signal circuit element is operativelyplaced on said <110> crystalline axis. In some cases, the thermalcompensator comprises a first pair of piezoelectric (PZT) circuitslocated about 180 degrees apart and wherein said pressure signal circuitelement comprises a second pair of PZT circuits located about 180degrees apart, and may be further limited such that at least one of saidfirst pair and said second pair include a circuit element selected fromthe group consisting of a half bridge and a Wheatstone bridge. In somecases where the pressure sensitive element comprises a diaphragm, thethermal compensator comprises at least one piezoelectric circuit placedon said non-recessed portion, wherein said pressure signal circuitelement comprises at least one piezoelectric circuit operatively placedon said diaphragm.

Regarding the packaging, in some cases at least one of said base layer,said mid-layer or said capping layer comprises a B-stage polyimide. TheB-stage polyimide can comprise a conductor-clad laminate which may be acopper-clad laminate. A compliant layer may be attached to said baselayer on the side of said base layer opposite to that of said mid-layer.

A variety of alternates may be included with the basic embodimentsdescribed. For example, at least one electric pad may be operativelyattached to said electric circuit. The sensor element may includes atleast a second recessed portion, wherein an electric pad of said atleast one electric pad is affixed within said second recessed portion.The sensor elements may include means for connecting said plurality ofsensor elements together and such means for connecting may comprise aflexible interconnect, including an electrical trace, which may beembedded in polymer. The flexible interconnect may comprises a metaltrace on a silicon spring. The thermal compensator can be an elementspatially separated from said sensor element. If the pressure sensorelement is a cantilever, it can be formed by anisotropic etching ofsilicon. The invention includes methods of manufacturing all of theembodiments and variations described above.

Another exemplary embodiment of the invention is a contact stress sensorincluding a package having a thickness no greater than 300 μm; and asensor element within said package, wherein said sensor elementcomprises: an elastic body including a recessed portion and a pressuresensitive element, wherein said pressure sensitive element extends overat least a portion of said recessed portion and is in contact with saidpackage; means for sensing pressure, wherein said means for sensingpressure is operatively connected to said pressure sensitive element;and means for providing thermal compensation to said means for sensingpressure. The means for sensing pressure can comprise an electricalcircuit which generally includes a piezoresistor material. In anotherembodiment, the means for sensing pressure comprises a capacitivesensor. The sensor element may have a thickness no greater than 280 μm.The package may be flexible or rigid. The rigid package may comprise amaterial selected from the group consisting of ceramic, metal, andplastic. The plastic may be a hard plastic or an epoxy based plastic. Insome cases, the package has a thickness no greater than 100 μm. Theflexible package may comprise a material selected from the groupconsisting of polymide, silicone and mylar.

Many variations apply to this embodiment also. For example, the pressuresensitive element may comprise a configuration selected from a groupconsisting of a cantilever and a diaphragm. The elastic body maycomprise silicon. The elastic body may comprise a thickness no greaterthan 100 μm. The pressure sensitive element may also comprise silicon.The pressure sensitive element may comprise a thickness no greater than50 μm. The sensor element may have a thickness no greater than 55 μm.The package can be configured to eliminate load risers to maintain aconstant thickness. The means for providing thermal compensation can beselected from the group consisting of a full wheat-stone bridge, a halfwheat-stone bridge and a temperature device. The package can comprise amaterial that is stretchable to conform to the surface of an object thathas a complex curvature. Additional contact stress sensors can becombined together and may or may not be formed into a one or twodimensional array. A variety of flexible and rigid interconnects aredisclosed to connect the contact stress sensors. See “MEMS ContactStress Sensing”, by Jack Kotovsky, Dissertation, Doctor of Philosophy inMechanical Engineering, University Of California, Davis, incorporatedherein by reference.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated into and form a partof the disclosure, illustrate embodiments of the invention and, togetherwith the description, serve to explain the principles of the invention.

FIG. 1A shows a cutaway view of an embodiment of the present inventionin the form of a single packaged contact stress sensor.

FIG. 1B shows an embodiment that includes a solder shim layer to assurethat the bottom side is uniformly supported and a controlled solderthickness is achieved.

FIG. 1C shows another embodiment that includes a solder shim layer toassure that the bottom side is uniformly supported and a controlledsolder thickness is achieved.

FIG. 1D shows a package that uses no adhesives, and is formed of b-stagepolyimides that are directly bonded together.

FIG. 2 shows a cutaway side view of the elastic body of FIG. 1A.

FIG. 3A is a scaled up version of a bottom view of an embodiment of aflexible body.

FIG. 3B is a magnified view of the boron traces on the bottom of theflexible body, near the electrical sensing circuits and thermalnormalizing circuits.

FIG. 3C shows the orientation of the <110> axis and the <100> axis theelastic body of FIG. 3A.

FIG. 3D shows how to connect the electrical sensing circuits and thermalnormalizing circuits in a wheatstone bridge configuration.

FIG. 4 shows a photograph of a cross-section of a 700 μm diameter by 15μm thick diaphragm in a 50 μm thick silicon ship with a gold overlay onthe back side of the diaphragm.

FIG. 5 shows a photograph of the cross-section of a 15.34 μm thickdiaphragm in a 48.80 μm thick silicon chip.

FIG. 6A is a side cutaway drawing of a non recessed elastic body withits electrical traces extending from the EB.

FIG. 6B is a side cutaway drawing of a recessed elastic body with itselectrical traces extending from the EB.

FIG. 7A shows a photograph of a number of individual elastic bodieslocated in a large area multi-layered package.

FIG. 7B shows the component layers of a multi-layered package similar tothe one shown in the photograph of FIG. 7A.

FIG. 8A shows a picture of an array of 64 diaphragms (on a dime) asindicated by the gold dots on the un-etched side of the silicon device.

FIG. 8B shows the etched side (on a dime) of the array of FIG. 8A.

FIG. 9 shows an exemplary electronic control and data acquisition systemfor an 8×8 array.

FIG. 10 illustrates a technique for enabling conductive traces to crosson the MEMS elastic body.

FIG. 11 shows the use of a polyimide bump to enable non-electricalcontact crossing of electrical traces.

FIG. 12 is a picture of the use of a polyimide bump to enable two tracesto cross without making contact.

FIGS. 13A-C shows several types of interconnection configurations.

FIG. 13D shows a metal on silicon-spring interconnect embedded in apackage.

FIG. 13E shows a free-standing metal interconnect with elastic bodies ateach end, all embedded in a package.

FIG. 14 is a picture of an unframed array with each row and columnsoldered to a single flexible cable.

FIG. 15 is a picture of a flexible array.

FIG. 16 is a picture showing an unetched element next to an etcheddiaphragm.

FIGS. 17A-I illustrate steps for producing an embodiment of thediaphragm type elastic body of the present invention.

FIGS. 18A-J illustrate steps for producing an embodiment of thediaphragm type elastic body of the present invention where theelectrical contact pads are extended from the chip.

FIGS. 19A-J illustrate steps for producing another embodiment of thediaphragm type elastic body of the present invention where theelectrical contact pads are extended from the chip.

FIGS. 20A-K illustrate a process for fabricating an elastic body thatincludes a recessed electrical connection to electrical contact padsthat are extended from the chip.

FIGS. 21A-M illustrate another process for fabricating an elastic bodythat includes a recessed electrical connection to electrical contactpads that are extended from the chip.

FIG. 22 shows a reduced chip area design where the contact pads arelocated on the chip.

FIG. 23 shows a design where the pads are located off of the chip.

FIG. 24 shows a picture of a pressure sensitive element comprising abeam supported at its ends.

FIG. 25A shows a bulk silicon (100) wafer patterned with thermal oxideand photoresist.

FIG. 25B shows the formation of a rib from a DRIE etch process.

FIG. 25C shows the photoresist has been removed and the (111) planes onwhich a KOH etch is to be performed.

FIG. 25D shows the resulting beam following the KOH etch.

FIG. 26 shows a sensor array configuration where the flexibleinterconnects are metal traces on silicon springs.

FIG. 27 is a view of a metal on silicon spring array.

FIG. 28 is a view of independent islands of silicon that each carry asensor arranged in a two-dimensional array.

FIG. 29 is a view of a 30×30 (900 sensor) array.

DETAILED DESCRIPTION OF THE INVENTION

The invention is a contact stress sensor configured as (i) a singlecontact stress sensor, (ii) a collection of single contact stresssensors in a common package (paddle), (iii) a one or two dimensionalarray of contact stress sensors without a package and (iv) a one or twodimensional array of contact stress sensors with a package.

I. Single Contact Stress Sensor Embodiments

A. Basic Multi-Layered Sensor Embodiment:

FIG. 1A shows a cutaway view of an embodiment of the present inventionin the form of a single packaged contact stress sensor. It includes amulti-layered package with an embedded sensor element, often referred toherein as the elastic body. The multi-layered package includes a baselayer 10, patterned electrical traces 12 on the base layer 10, solderlayer 14, a first coverlayer portion 16 that includes a cutout sectionfor the elastic body 22 and a second coverlayer portion 24, sometimesreferred to herein as a capping (or cover) layer. The base layer 10 ismade of polyimide in several embodiments. In some embodiments, thepatterned electrical traces 12 are formed from copper either bonded tothe polyimide or adhered to the polyimide with an adhesive. Adhesivelesspackages are the most stable. In an adhesiveless package, the copper isapplied to the polyimide directly without the use of visco-elasticadhesives. A combination of adhesive material and polyimide may be usedto fabricate a solder shim layer (25 in FIG. 1B). A b-stage polyimidemay also be used that has no adhesives, the polyimide layers aredirectly bonded to each other. For example, in FIG. 1B, the adhesivelayers could be replaced with polyimide layers. This embodiment showsfirst coverlayer portion 16 as comprising two layers; a first layer 18is made of adhesive material and a second layer 20 is made of polyimide.The second coverlayer portion 24 comprises two layers; a first layer 26is made of adhesive and a second layer 28 is made of polyimide. Atvarious places within this document, the layers that comprise polyimidein this embodiment are referred to as the package layers and theparticular material used may be referred to as the package material. Forexample, the package material in the embodiment of FIG. 1A is polyimide.Note that the elastic body 22 includes a pressure sensitive element(PSE), discussed in detail below. Exemplary embodiments of the PSE maybe in the form of a diaphragm or a cantilever. An electrical sensingcircuit is located on the PSE. Embodiments of PSEs and electricalsensing circuits are discussed in detail infra. FIG. 1C shows anotherversion of the package that keeps adhesive out of the back of thediaphragm. Like components of this figure and FIG. 1B are identicallylabeled. Notice that adhesive layer 26 includes an opening such that theelastic body 22 makes contact with polyimide layer 28. Notice also thatthe total thickness of layers 18, 20 and 26 of FIG. 1C is equal to thetotal thickness of just layers 18 and 20 of FIG. 1B.

Referring again to FIG. 1A, electrical traces 23, on the elastic body22, which in this embodiment includes a Micro-Electro-Mechanical Systems(MEMS) diaphragm, are soldered to the electrical traces 12 within thecutout section of the multilayered package. The elastic body includes astructural frame, a sensitive element (diaphragm or cantilever) and theembedded circuit. In other embodiments, the elastic body may have adifferent configuration, such as a cantilever. The elastic body in allembodiments is a MEMS device.

FIG. 1B shows an embodiment that includes a solder shim layer 25 toassure that the bottom side (shown as reference number 29 in FIG. 2), isuniformly supported. The solder shim layer also ensures a deterministicsolder thickness is achieved underneath the chip to avoid solder jointfailure. Too little solder can result in solder joint failures. Elementsidentical to those of FIG. 1A are numbered identically. The solder shimlayer 25 is made of two sub layers: a first layer 27 is made of adhesivematerial and a second layer 29 is made of polyimide. Solder 31 can beelectroplated up through solder shim layer 25, and excess solder canescape beyond the edge of the chip prior to coverlayer 16 being applied.The solder shim layer can be laser cut to create the hole for the solderto electroplate up through.

FIG. 1D shows a package that uses no adhesives, and is formed of b-stagepolyimides that are directly bonded together. On base layer 10 is formedcopper trace 12 which is electrically connected to the elastic body 22via solder 14 The diaphragm part of the chip is preferably not bonded tothe copper trace 12 so that shear loads are not coupled into thediaphragm. It is desirable that the package be free to translaterelative to the diaphragm if needed so that the diaphragm only seesperpendicular loads, not shear loads. The elastic body is packaged inthe opening in a mid layer (coverlayer) 5 over which is attachedcoverlayer 6. Residual stress curl of the multilayer package may berelieved with a symmetrically designed package (e.g., layers 10 and 6are matched in thickness) to cancel stresses.

FIG. 2 shows a cutaway side view of elastic body 22 of FIG. 1A. Theelastic body is fabricated from a solid body of single crystal silicon.A recessed portion 30 is formed in elastic body 22. The recessed portionin the embodiment shown would be seen as circular if viewed from thetop; however, it can be formed as other areas, including rectangular andsquare. The pressure sensitive element 32 is a thinned area near thebottom side 29 of elastic body 22. This element can be in the form of adiaphragm, as in the embodiment of FIG. 1A and FIG. 2 or it can be acantilever as discussed below. The diaphragm can be sealed (with top andbottom, creating a reference volume), or it can be a single unsealeddiaphragm, as in this embodiment. A load amplification element (e.g., anelectroplated bump), as discussed below, can be included with either thediaphragm or the cantilever embodiments. Electrical sensing circuits andthermal normalizing circuits, discussed below, are typically bothincluded on pressure sensitive element 32. These circuits may be formedfrom a single resistor, a pair of resistors (a half bridge) or awheatstone bridge (a full bridge of 4 resistors).

The relative dimensions of the elements of FIG. 1A are not presented toscale. FIG. 3A is a scaled up version of a bottom view of an embodimentof a flexible body 40. This view would roughly correspond to a bottomview (at 29) of FIG. 2, although the scales are different, especiallythat of the recessed portion 30 of FIG. 2 The area within the centralpart of dashed line 42 is the pressure sensitive element of thisembodiment. The dashed line 42 shows the outline of the recessedportion, which is recessed from the opposite side. Some details of theelectrical features of the flexible body are shown in this view.Metallization traces 44-47 include contact windows 48-51, respectively.The contact windows are the locations where the metal pads 44-47electrically communicate with the doped traces 52-55. The metal pads44-47 are soldered to the electrical traces 12 (FIGS. 1A and 1B).

FIG. 3B is a magnified view of the boron traces on the bottom of theflexible body, near the electrical sensing circuits 56 and 58 andthermal normalizing circuits 60 and 62. As part of the fabricationprocess, the boron traces 52-55, electrical sensing circuits 56 and 58and thermal normalizing circuits 60 and 62 are implanted into thesilicon flexible body. Annealing activates the piezoelectric propertiesof electrical sensing circuits 56 and 58 and thermal normalizingcircuits 60 and 62. Electrical sensing circuits 56 and 58 are placed onthe <110> crystalline axis of the elastic body and thermal normalizingcircuits 60 and 62 are placed on the <100> crystalline axis. Theelectrical response depends on the crystallographic orientation. Traceson the <100> axes have minimal response to strain and traces on the<110> axes have maximal response to strain for p-type silicon. FIG. 3Cshows the orientation of the <110> axis and the <100> axis the elasticbody of FIG. 3A. The thermal normalizing circuits are placed on axesthat show little strain sensitivity but sit on the diaphragm to respondthermally identically to the stress sensing circuit. The thermalnormalizing circuit may also be placed in the bulk silicon (not on thediaphragm) and their orientation may be altered. FIG. 3D shows how toconnect electrical sensing circuits 56 and 58 and thermal normalizingcircuits 60 and 62 in a wheatstone bridge configuration.

The base layer (BL), which is layer 10 in FIG. 1A, is the foundation ofthe sensor package. In this embodiment, it provides electrical conduitsto the elastic body (sometimes referred to as the “die”) to connect theelectrical circuits thereon with the outside world. It must accomplishthese electronic goals and also serves other functions. The BL, andpackage as a whole, electrically isolates the sensor from the outsideenvironment to avoid fouling of the electrical signals. Extreme versionsof this include use in vivo or in fluid environments. The BL and packagealso mechanically protect the die by providing a polymeric coating toisolate the die from damaging environments. The BL is very thin (someembodiments have a thickness within a range from 12-50 μm) to help forma complete system of minimal thickness. Finally, the BL is flexible toaccommodate various applications. Embodiments of the BL are a polymerfilm (e.g., Polyimide; which may be used from the brand name KAPTON)with patterned metal traces (e.g., gold or copper) on the film. Otherfilms, flexible and rigid, may be used for the BL or cover layers.

Patterned electrical traces 12 are located on BL 10 as shown in FIG. 1A.These conductive traces can be metal or other conducting materials(e.g., patterned polymers). The trace geometries are typicallyphotolithographically defined. Screen printing and other methods may beused in their definition. Conductive patterns have dimensions as smallas 25 μm but are typically larger. The conductive traces electricallycommunicate with electrical traces 23 on elastic body 22 through directcontact, bonds or solder joints. Signal carried on the conductive tracescan be brought to a circuit board, a connector or other hardware.

To enable a package that maintains a uniform thickness, a cover layer(CL) 16 is implemented that acts as a shim or a standoff in themulti-layer package. The sensor has a finite thickness (typically 50 μmbut thicker and thinner sensors are possible). When mounted on the BL10, it rests above the plane of the BL. If the cover layer did not havean opening into which the elastic body were to be emplaced, and acapping layer, such as cover layer 24, was simply placed over theelastic body and BL, the region of the package at the elastic body wouldbe thicker (bulge) than the surrounding regions (the elastic body is asmall element on a larger BL). Further, if such a package were placedbetween stiff, flat surfaces, all the load would focus on the elasticbody as it would be thicker than the rest of the package. The loadfocusing or perturbation would introduce significant error in themeasurement for which the elastic body is designed. To avoid thiseffect, which is herein referred to as a “load riser”, the thickness ofthe package can be maintained by including material of matched thicknessplaced adjacent to the elastic body. This is a cover layer (CL)(sometimes referred to herein as the mid-layer) with cutouts (the firstCL is referred to in FIG. 1A as cover layer 16 and comprises two sublayers 18 and 20). An opening is defined in CL 16 (e.g., defined bylaser cutting or stamp cutting) that is closely matched to the perimetershape of the sensor and similar in thickness. Typically the twosub-layers of CL 16 are a KAPTON layer (layer 20) combined with anadhesive layer (layer 18) that bonds CL 16 to BL 10. Typical adhesivesinclude epoxies and acrylic adhesives. An adhesiveless package,described infra, has also been designed that uses a polyimide-polyimidebond through the use of B-stage polyimides (brand name ESPANEX).

The elastic body must electrically communicate with the conductivetraces. This is most commonly accomplished with use of a solder joint.As the thickness of the packaged sensor and the loading of the EB arecritical to the accurate function of the sensor system, the soldergeometry must be very carefully defined. Arbitrary buildup of soldermaterial under the EB would be detrimental to sensor function. Thesolder thus must achieve electrical communication between the traces andthe EB but must maintain strict geometric constraints. Its thickness canbe controlled through electroplating or other methods (e.g., solderleveling, solder dipping, preforms, etc.). The solder must also beadequately strong to sustain stresses that exist between the package andthe EB (e.g., thermal strains due to Coefficient of Thermal Expansion(CTE) mismatch between the EB and BL).

Once mounted on BL 10, the EB 22 can be mechanically protected byentirely encapsulating it in the multi-ply package. The capping or coverlayer 24 is applied over the non-trace side of the EB 22 and can bebonded to the layer beneath it to entirely capture and encapsulate theEB. CL 24 is laminated to CL 16 (or to the BL if desired in someapplications) e.g., with adhesive as described above, except in theadhesiveless package described infra. CL 24 typically comprises two sublayers; the outer layer is a polyimide layer 28 and the under layer isan adhesive layer 26.

Silicon is selected as the material for elastic body 22 due to itsexcellent material properties, machinability, electrical properties andwell characterized behavior. It is a semi-conductor that can beselectively doped, or treated, to behave as a piezoresistor. Asdiscussed above, the traces in some embodiments are formed by implantingboron into the silicon and then annealing it so it functions as apiezoresistor. The overall form of the elastic body is a very thinstructure with an etched diaphragm in certain embodiments or acantilever in other embodiments. FIG. 4 shows a photograph of across-section of a 700 μm diameter by 15 μm thick diaphragm in a 50 μmthick silicon chip with a gold overlay on the back side of thediaphragm. The gold overlay may be of arbitrary thickness to concentrateloads on the diaphragm thus enhancing device sensitivity. FIG. 5 shows aphotograph of the cross-section of a 15.34 μm thick diaphragm in a 48.80μm thick silicon chip. This diaphragm was formed in a standard siliconwafer. SOI wafers may also be used to facilitate diaphragm thicknesscontrol.

To accurately achieve interface load measurements, the overall sensorsystem must be very thin. Typically, the thinner the sensor, the lessthe sensor will perturb the measurement of interest. Creating aload-sensing device of membrane thickness is a very interesting anddesirable feature of the present single-sensor invention. Advances insilicon processing and packaging have allowed for this accomplishment.Silicon devices of arbitrary thickness can be created. A processdescribed below has reproducibly produced EBs with thicknesses as littleas 10 microns. Even thinner devices are possible but requiremodification to the process. The EBs can be formed up to largethicknesses (e.g., 500 microns) if desired.

The sensor functions by relating applied loads to electrical output.This is accomplished by using the applied loads to deform the pressuresensitive element on the EB. The pressure sensitive element, for thediaphragm type sensor, is formed by etching a blind hole in the EB toleave a thinned silicon diaphragm. Other methods may be used to form therecessed body including a 2-step anisotropic etching method. For anembodiment of the diaphragm pressure sensitive element, the recessedportion is formed by removal of silicon material to sculpt the shape ofthe body. The removal process may be a wet or dry chemical etch. Dryetching (e.g., by plasma) is often desirable because it is anisotropicand well controlled. Etch recipes have been formed that leave verysmooth surfaces of silicon where material has been removed. Non-smoothsurfaces negatively affect mechanical toughness because they can act asinitiation sites for fractures. Therefore, the smoothness of thebackside of the deforming load sensitive element is an importantconsideration as well as the sides of the chip and blind hole that mayact as crack initiation points.

Excellent control and process robustness can be achieved by using astopping layer to define the recessed portion. By using asilicon-on-insulator (SOI) wafer, the insulating layer acts as an etchstop to leave the device layer silicon unetched. Purchase of precisionSOI wafers pre-determines the load sensitive element's thickness.

The pressure sensitive element is a structure that deforms with appliedloads that are being measured. The deformation produces material strain.Piezoresistive traces are strategically placed at areas of high strainto produce large resistance change as a function of strain. Byelectrically monitoring the resistance values, the applied loads can bemeasured as device resistance is directly proportional to applied loads.For a circular diaphragm shaped pressure sensitive element, theload-sensing resistors are placed at the outer radius of the diaphragmwhere bending strains are greatest. This optimizes signal to noiseperformance of the device.

For silicon, the crystallographic axes determine the piezoresistivebehavior of the material. For p-type resistors, the <110> directionsgive greatest load sensitivity. The load sensitive elements doped intothe EB are oriented radially inward on the diaphragm along the <110>direction in some embodiments. Some of these embodiments include fourresistive elements in the EB to permit thermal compensation. Twoelements measure strain (and are thermally sensitive) and two elementsare included to measure temperature only. The temperature only elementsare positioned in the unstrained EB or at the diaphragm but on the <100>directions where there is no significant piezoresistive response tostrain. This is a configuration referred to both herein and in the art,as a standard Wheatstone bridge configuration.

B. Basic Multi-Layered Sensor without Solder Shim Layer Embodiment:

An embodiment of the invention is the same as the device describedabove, except that it does not have the solder shim layer. The soldershim layer provides a thick solder connection between the elastic bodyand base layer, which helps long term reliability. It alleviatesthermally induced stresses resulting from different coefficients ofthermal expansion between the elastic body and base layer. The soldershim layer does this while maintaining critical load continuity with thepressure sensitive element.

C. Basic Multi-Layered Sensor with Load Dot Embodiment:

This embodiment includes a load dot on top, on bottom or on top andbottom of the base layer (10 in FIG. 1). This load dot is placed in linewith the pressure sensitive element and helps transmit stress to thepressure sensitive element. The load dot helps transmit the stressapplied by a flat surface to the pressure sensitive element. Without theload dot, the pressure sensitive element might not be deformed by thestress applied, especially against very stiff surfaces. The load dotprovides a geometric feature that accentuates load over the sensingelement.

D. Basic Multi-Layered Sensor with a Compliant Layer Embodiment:

In this embodiment, the multi-layered package includes a compliant layer(e.g., silicone rubber put on the underside of the base layer). Theadded compliant layer assists with the transmission of stress to theelastic body. The pressure sensitive element deforms slightly withapplied loads. As the sensor structure around the pressure sensitiveelement bears load but is not compliant, it may shield the pressuresensitive element (PSE) from the load. This means the sensor will notfunction if the load is not transmitted to the PSE. This problemsurfaces most when a very stiff surface contacts the sensor (e.g.,applications with stiff surfaces like in the head-gasket of anautomobile engine). If the surface is adequately stiff, its appliedforce will only load the surrounding structure once the PSE has deformedbeneath the plane of the surrounding structure. The addition of acompliant structure between the PSE and the surface applying load (e.g.,a thin membrane of a rubber-like material between the PSE and the hardsurface), ensures that load continues to be transmitted to the PSE evenonce deformed from that load. The sensor's response depends on thehardness of the surface it is in contact with. As the surface grows inhardness, use of an added compliant surface at the interface with thesensor can augment its function and accuracy. The compliant surface cancover the entire sensor package or be a small structure placed over thePSE only.

E. Basic Multi-Layered Sensor with B-stage Polyimide Embodiment:

FIG. 1D shows a device that has no adhesive between the polyimidelayers. Thus, all the elements are identical or similar to those of FIG.1A, except that cover layers 16 and 24 of FIG. 1A do not consist ofsub-layers. The basic sensor package can exist with or without thesolder shim layer. This changes the material used in the cover layer tostrictly polyimide, free of adhesives. The elimination of adhesivematerial from the package permits operation at higher temperatures andenhances dynamic performance of the device (polyimide is more elastic;the adhesives have known visco-elastic limitations).

F. Solder Free Multi-Layered Sensor Embodiment:

For this configuration, the elastic body is placed into the cutout inthe cover layer (16 in FIG. 1A) and it is not soldered into place. Thecover layer 24 then presses the elastic body down to make contact withthe gold traces. This version of the device does not use solder. Theelectrical communication between the elastic body and the package isachieved through direct contact of conducting surfaces (e.g., gold togold contact). Solder produces a variability of thickness in the packagewhich can increase the variability in measurements between sensors,particularly in stiff applications. Also, eliminating the soldereliminates coefficient of thermal expansion (CTE) mismatch between tilesilicon MEMS and the polyimide package. The solder joint welds theelastic body to the package and the two materials show different thermalresponse. At extreme temperatures, this may introduce an error in thesensor's readout or induce solder failure. By mechanically decouplingthe elastic body from the package, the thermal differences will nolonger affect sensor performance.

G. Some Variations for Basic Multi-Layered Sensor Embodiment:

In this type of sensor, the solder shim layer is optional and the coverlayer made of adhesive material is optional. If the cover layer isomitted, a capping layer can still be used. One can still make a recessfor the chip to have a constant thickness, or one can accept anon-uniform thickness. One can also omit the capping layer. In thiscase, the elastic body may or may not protrude above the capping layer.So, one can have both a uniform and non-uniform thickness design.

H. Simple Single-Layered Sensor Embodiment:

This single-layered package consists of a base layer and patternedelectrical (copper) traces on the base layer. The EB is attached bysoldering it to the traces on the base layer. The EB can be attached tothe package, i.e., it can be electrically connected to the coppertraces. The EB may be joined to a thin package base layer (e.g.,KAPTON), a circuit board (e.g., an FR4 board) or any surface of anythickness (e.g., the sidewalk to detect passersby for use as asurveillance device.

I. Basic Off Chip Connect Multi-Layered Sensor Embodiment:

The elastic body consists of a solid body, and pads extending off thesolid body. The pads can be on photolithographically defined polyimidewhich is attached to the solid body. This is mainly a variation in theconnection between the conductive traces of the sensor and its package.Instead of having the electrical pads on the solid body, the design putsthe pads off the body at a distance using photolithographically definedfree-standing metal traces or metal traces supported by polyimide (orother materials) attached to the solid body. Establishing the solderjoint off the chip allows the chip to be smaller and alleviates concernsof solder thickness variability under the chip.

The bond pads extending off the body can be recessed and non-recessed.For all the on-chip pad designs described above, the electricalcommunication between the silicon device and the outside world wasestablished by metal pads that sit on the EB. These pads on the EBcommunicate with conductive traces on the package. As the EB held themetal contact pads, the EB was forced to be of a size large enough tohouse the pads. This also forces the electrical communication to beachieved on the same face of the EB as the pressure sensitive element(PSE). This mechanical constraint limits the performance of the deviceas factors like solder thickness can impact the load conduction to thePSE. The pads can be placed off the chip by extending the metal off theEB. This allows the EB to be greatly reduced in size, offering a varietyof advantages. The smaller EB is stiffer allowing it to be thinner.Smaller and thinner EB's extend the applications that the sensor can beused in. Removal of the solder from the face of the chip that containsthe PSE improves device accuracy and reduces variability betweendevices. The total thickness of the package can be reduced with off-chippads as well.

The metal can be extended off the chip by patterning the metal beyondthe physical limits of the EB. This requires unique processing to allowthe freestanding metal to exist beyond the support of the EB. Use offrames and tethers are design solutions to this problem. The metal canbe patterned to exist in a single plane. In this case it simply extendsbeyond the edge of the EB. Alternatively, the metal may be led off theedge of the EB in a recessed slot. The recessed slot avoids shearing ofthe metal at the EB edge. This is a mechanism to reduce metal failurewith loads applied.

FIG. 6A is a side cutaway drawing of a non recessed elastic body 70 in apad-off chip design or an element of a two-dimensional array. The sketchshows the electrical traces 74 and 75 sitting on supporting polyimide 72and 73. The solder joint between these traces and the conductive traceson 76 is not shown in these sketches. The EB is fully enclosed due toits placement in the opening formed in layer 78 and on capping layer 80.Force shown by lines 82 can cause a shearing of the traces at the edgeof the elastic body. FIG. 6B is a side cutaway drawing of a recessedelastic body 90 with electrical traces 94 and 95 extending from the EBand in contact with and supported by polyimide layers 93 and 93. This isa pad off chip design or an element of a two-dimensional array. The EBis fully enclosed due to its placement in the opening formed in layer 98and on capping layer 100. Force shown by lines 102 have fess shearingeffect on the traces at the edge of the elastic body.

II. A Collection of Single Contact Stress Sensors in a Common Package(e.g., a Paddle)

A. Multi-Layered Package with Multiple Stress Sensors Embodiment:

In this embodiment, as shown in FIG. 7A, a multi-layered packageincludes a base layer that extends over a larger area and canaccommodate multiple elastic bodies. The patterned electrical tracesconnect to multiple elastic bodies. The cover layer has cutouts formultiple elastic bodies. Each elastic body can be attached by solderingit to the traces in the cutouts in the cover layer. All versions of thesingle sensor described above can be implemented as a collection ofsingle sensors. The package is arbitrary in shape. It can appear as athin strip or a large surface. If a number of measurement points aredesired over a surface (e.g., loads at various points on a car seatunderneath the driver's pelvis), the package can be sized to accommodatethe application (e.g., the shape of a car seat). The individual sensorsmay be accurately placed at any location on this surface. For example,15 sensors may be deployed on the seat surface at specific locationsunder the bony prominences of the driver's pelvis. This is notconsidered herein as an array of sensors but instead as an arbitrarycollection of single sensors integrated within a single sensor package.The outputs of the sensors can be at a common connector. FIG. 7A shows aphotograph of a number of individual elastic bodies located in a largearea multi-layered package. FIG. 7B shows the component layers of amulti-layered package similar to the one shown in the photograph of FIG.7A. A base layer 110 with circuits 112 and through holes 114 (which aresoldered to connector 3) is located between a bottom cover layer 116 andcover layer 118. Backside coverfilm 116 covers a metal pattern (that isnot visible in this view) on the underside of 110. This underside metalpattern is a ground plane that helps reduce electrical noise on thecircuit. Use of a ground plane is a circuit design option implemented inembodiments of this flex circuit assembly. A top cover layer 120 isplaced over cover layer 118. Cover layer 118 includes cutout sections122 at various places for location of an elastic body.

The above described elastic bodies and variations thereof can beutilized in arrays that are unpackaged or packaged. Exemplaryembodiments, interconnection and fabrication techniques are providedbelow.

FIG. 8A shows a picture of an array of 64 diaphragms (on a dime) asindicated by the gold dots on the un-etched side of the diaphragm. Thegold dots are used to indicate the location of the diaphragm. FIG. 8Bshows the etched side (on a dime) of the array of FIG. 8A. The totalchip thickness is 50 μm. FIG. 9 shows an exemplary electronic controland data acquisition system for an 8×8 array. FIG. 10 illustrates atechnique for enabling conductive traces to cross on the MEMS elasticbody 220 as needed to accomplish the circuit shown in FIG. 9. Aconductive trace 222 on elastic body 220 is covered with a polyimidebump 224, which is then covered with the crossing conductive trace 226.FIG. 11 a top view of the crossing shown in FIG. 10 in cross-section,shows the silicon outline 230, the diaphragm outline 232, metal trace234 on silicon, polyimide bump 236 that bridges over metal trace 234 andmetal trace 238 which is laid over polyimide bump 236 to cross withouttouching metal trace 234. FIG. 12 is a picture of the use of a polyimidebump to enable two traces to cross without making contact, a physicalimplementation of FIG. 11. FIGS. 13A-C show several types ofinterconnection configurations. The interconnect 240 of FIG. 13A willallow for greater flexibility between connected diaphragms thaninterconnect 252 of FIG. 13B, which in turn will allow for greaterflexibility than interconnect 244 of FIG. 13C. Each interconnect isformed of an electrical trace embedded in polyimide. FIG. 13D shows ametal 250 on silicon-spring 251 interconnect embedded in a package 252.FIG. 13E shows a free-standing metal interconnect 253 with silicondiaphragms 254 and 255 at each end, all embedded in package 256. Amotivation for the freestanding metal interconnect (13E) is to allowgreater flexibility than 13D although 13D does not suffer from the metalshearing problem discussed earlier as the metal is entirely supported bysilicon.

FIG. 14 is a picture of an unframed array with each row and columnsoldered to a single flexible cable. FIG. 15 is a picture of a flexiblearray. FIG. 16 is a picture showing an unetched element 260 next to anetched diaphragm 262. When an array uses diaphragms that do not includethe use of the <100> axis of the silicon diaphragm to correct forthermal effects on the signal, the signal measured on this unetchedelement shown in FIG. 16 is usable to cancel the thermally inducedsignal. It also can be used to measure temperature at an arbitrarynumber of locations (i.e., a temperature mapping array). Although thesedesigns show arrays of contact stress sensors, any silicon device may bearrayed with this interconnection scheme (a temperature sensor orother).

FIGS. 17A-I illustrate steps for producing an embodiment of thediaphragm type elastic body of the present invention. This embodimentincludes the electrical contact pads on the chip, as e.g., shown in FIG.22 discussed below. FIG. 17A shows the starting wafer configuration. Inthis example, the starting wafer includes a silicon handle 300 (e.g.,˜500 μm thick), a BOX layer 302 (e.g., 0.2 μm thick), a silicon devicelayer 304 (e.g., varying thicknesses: 10, 15, 20, 25 μm), and maskingoxide layer 306 (e.g., 0.1 μm). FIG. 17B shows boron 308 implanted intothe silicon device layer to form the resistors. FIG. 17C shows theresults of etching through the masking oxide layer 306 to form thecontact holes 310 to communicate with the boron implants. Holes 310 areetched through the masking oxide layer and include etches in the streetsbetween adjacent dies. FIG. 17D shows the metal contacts 312 and 314formed by evaporation and patterning of metal (e.g., Ti/Ni/Au or other).The metal 316 covering the diaphragm is used solely to help visualizethe diaphragm location from the top of the device and to potentiallybuild a load concentrating ‘bump’ on the chip as previously describedfor the package. The original silicon handle 300 of FIG. 17A is thinnedto ˜40 μm to form handle 318 as shown in FIG. 17E. FIG. 17F shows theresult of attaching the thin silicon wafer, upside down, to a quartzhandle 320, to allow further processing without breaking the siliconwafer and to facilitate the backside etching. FIG. 17G shows the resultsof etching through the silicon handle (from the backside), stopping atthe BOX layer, to form the diaphragm. FIG. 17H illustrates the step ofetching through the silicon handle (from the backside), the BOX layer,and the silicon device layer to form the streets 322 and 323 betweenadjacent dies, allowing individual dies to be singulated from the wafer.FIG. 17I shows the released silicon device after release from the quartzhandle.

FIGS. 18A-J illustrate steps for producing an embodiment of thediaphragm type elastic body of the present invention where theelectrical contact pads are extended from the chip, as e.g., shown inFIG. 23 discussed below. FIG. 18A shows the starting waferconfiguration. In this example, the starting wafer includes a siliconhandle 400 (e.g., ˜500 μm thick), a BOX layer 402 (e.g., 0.2 μm thick),a silicon device layer 404 (e.g., varying thicknesses: 10, 15, 20, 25μm), and masking oxide layer 406 (e.g., 0.1 μm). FIG. 18B shows boron408 and 409 implanted into the silicon device layer to form theresistors. FIG. 18C shows the results of etching through the maskingoxide layer 406 to form the contact holes 410 and 411 for the boronimplants. Holes 414 and 415 are also etched through the masking oxidelayer (at the polyimide bridge locations). FIG. 18D shows the resultsfrom spinning on and curing polyimide (e.g., ˜5 μm) to form the bridges416 and 418 for the metal lines. Open holes 420 and 422 in the polyimideform the contact holes for the boron implants. As shown in FIG. 18E, apattern of Ti/Ni/Au (or other metals) is then evaporated to form themetal contacts 424 and 426. The metal covering 428 on the diaphragm isused solely to help visualize the diaphragm location from the top of thedevice and potentially create a load accentuating bump over thediaphragm as previously described for the package. The silicon handle isthen thinned to ˜40 μm as shown in FIG. 18F. The thinned silicon waferis then attached, upside down, to a quartz handle 430, to allow furtherprocessing without breaking the silicon wafer and to facilitate thebackside etching. The silicon handle is then etched down to the stoppingoxide layer, as shown in FIG. 18H, to form the diaphragm. As shown inFIG. 18I, etching through the silicon handle (from the backside), theBOX layer, and the silicon device layer forms the polyimide bridges 432and 434 and the streets between adjacent dies, allowing individual diesto be released. FIG. 18J shows the silicon device released from thequartz handle.

FIGS. 19A-J illustrate steps for producing another embodiment of thediaphragm type elastic body of the present invention where theelectrical contact pads are extended from the chip, as e.g., shown inFIG. 23 discussed below. FIG. 19A shows the starting waferconfiguration. In this example, the starting wafer includes a siliconhandle 500 (e.g., ˜500 μm thick), a BOX layer 502 (e.g., 0.2 μm thick),a silicon device layer 504 (e.g., varying thicknesses: 10, 15, 20, 25μm), and masking oxide layer 506 (e.g., 0.1 μm). FIG. 19B shows boron508 and 509 implanted into the silicon device layer to form theresistors. FIG. 19C shows the results of etching through the maskingoxide layer 506 to form the contact holes 510 and 511 for the boronimplants. Holes 514 and 515 are also etched through the masking oxidelayer (at the polyimide bridge locations). FIG. 19D shows the resultsfrom spinning on and curing polyimide (e.g., ˜5 μm) to form the bridges516 and 518 for the metal lines. Open holes 520 and 522 in the polyimideform the contact holes for the boron implants. A polyimide bump 523 iscreated in the center of the diaphragm (used to planarize the surface,thus minimizing the stress on the polyimide bridges at the silicon edgesand preventing breakage of the metal lines and to accentuate load on thediaphragm). A pattern of Ti/Ni/Au (or other metals) is then evaporatedto form the metal contacts 524 and 526. The metal covering 528 on thediaphragm is used solely to help visualize the diaphragm location fromthe top of the device. The silicon handle is then thinned to ˜40 μm asshown in FIG. 19F. The thinned silicon wafer is then attached, upsidedown, to a quartz handle 530, to allow further processing withoutbreaking the silicon wafer and to facilitate the backside etching. Thesilicon handle is then etched down to the stopping oxide layer, as shownin FIG. 19H, to form the diaphragm. As shown in FIG. 19I, etchingthrough the silicon handle (from the backside), the BOX layer, and thesilicon device layer forms the polyimide bridges 532 and 534 and thestreets between adjacent dies, allowing individual dies to be released.FIG. 19J shows the silicon device released from the quartz handle.

FIGS. 20A-K illustrate a process for fabricating an elastic body thatincludes a recessed electrical connection to electrical contact padsthat are extended from the chip. The starting wafer includes a siliconhandle 600 (e.g., ˜500 μm thick), BOX layer 602 (e.g., 0.2 μm thick),silicon device layer 604 (e.g., thicknesses: 10, 15, 20, 25 μm), andmasking oxide layer 606 (0.1 μm). Boron portions 608 and 610 areimplanted in the silicon layer 604 (FIG. 20B) to form the resistors. Themasking oxide layer is etched through to form the contact holes 612 and614 for the boron implants (FIG. 20C). FIG. 20D illustrates the recesses616 and 618 for the polyimide bridges. Recesses 616 and 618 are formedby etching through the masking oxide layer 606 and into the silicondevice layer 604. Recesses 616 and 618 are used to recess the surfaceoff which the metal lines depart the EB, thus minimizing the stress onthe polyimide bridges at the silicon edges and preventing breakage ofthe metal lines. FIG. 20E shows results from the step of spinning on andcuring the polyimide (e.g., ˜5 μm thick) to form the bridges 620 and 622for the metal lines. Open holes 624 and 626 in the polyimide form thecontact holes for the boron implants. FIG. 20F shows the pattern ofTi/Ni/Au that forms the metal contacts 628 and 630. The metal covering632 on the diaphragm is used solely to help visualize the diaphragmlocation from the top of the device. The silicon handle is thinned to˜40 μm (FIG. 20G). FIG. 20H shows the results from attaching the thinsilicon wafer, upside down, to a quartz handle 634, to allow furtherprocessing without breaking the silicon wafer and to facilitate thebackside etching. FIG. 20I shows the silicon handle after it has beenetched through (from the backside), with the etch stopping at the BOXlayer, to form the diaphragm. FIG. 20J shows the results of etchingthrough the silicon handle (from the backside), the BOX layer, and thesilicon device layer to form the polyimide bridges and the streetsbetween adjacent dies, allowing individual dies to be released. FIG. 20Kshows the silicon device released from the quartz handle.

FIGS. 21A-M illustrate another process for fabricating an elastic bodythat includes a recessed electrical connection to electrical contactpads that are extended from the chip. In FIG. 21A, Starting wafer:Silicon Handle (˜500 μm) 700, BOX layer (0.2 μm) 702, Silicon DeviceLayer (Varying Thicknesses: 10, 15, 20, 25 μm) 704, and Masking OxideLayer (0.1 μm) 706. In FIG. 21B, etch through the masking oxide layerand into the silicon device layer to form the recesses 708 and 710 forthe polyimide bridges (used to planarize the surface, thus minimizingthe stress on the polyimide bridges at the silicon edges and preventingbreakage of the metal lines). In FIG. 21C, strip the masking oxidelayer. In FIG. 21D, thermally oxidize the silicon (0.1 μm) to provideoxid layer 712. In FIG. 21E, implant Boron 714 and 716 into the silicondevice layer to form the resistors. In FIG. 21F, etch the oxide to formthe contact holes 718 and 720 for the metal contacts to the Boronimplants. In FIG. 21G, spin on and cure the polyimide (˜5 μm) to formthe bridges 722 and 724 for the metal lines. Open holes in the polyimideto form the contact holes for the Boron implants. Create a polyimide“bump” 726 in the center of the diaphragm (to accentuate load to thediaphragm and enhance device response). In FIG. 21H, evaporate andpattern the Ti/Ni/Au to form the metal contacts 728 and 730. (The metallayer 732 covering the diaphragm is used solely to help visualize thediaphragm location from the top of the device.) In FIG. 21I, thin thesilicon handle to ˜40 μm. In FIG. 21J, attach the thin silicon wafer,upside down, to a quartz handle 734, to allow further processing withoutbreaking the silicon wafer and to facilitate the backside etching. InFIG. 21K, etch through the silicon handle (from the backside), stoppingat the BOX layer, to form the diaphragm. In FIG. 21L, etch through thesilicon handle (from the backside), the BOX layer, and the silicondevice layer to form the polyimide bridges 736 and 738 and the streetsbetween adjacent dies, allowing individual dies to be released. In FIG.21M, release the silicon devices from the quartz handle.

FIG. 22 shows a reduced chip area design where the contact pads 751-754are located on the chip. This embodiment also includes an etch 756 thatseparates the chip 750 from its handling frame 758. Breakaway tethers761-764 temporarily hold the chip to the handling frame to allow tweezerhandling of the frame. The handling frame includes an outer etch 759.

FIG. 23 shows a design where the pads 771-774 are located off of thechip 770. This design allows for excellent diaphragm load continuity,minimized die areas (e.g., 300 microns in diameter), a recessed designand includes a handling frame 776 with severable tethers 777-780.

Although the diaphragm embodiments have been extensively discussedabove, a cantilever beam or a beam supported at its ends can be used asthe pressure sensitive element in the present invention. FIG. 24 shows apicture of a pressure sensitive element comprising a beam 800 supportedat its ends. An exemplary process for making such a beam is shown inFIGS. 25A-D. FIG. 25A shows a bulk silicon (100) wafer 802 patternedthermal oxide (or silicon nitride) 804 and photoresist 806. FIG. 25Bshows the formation of a rib 808 from a DRIE etch process. FIG. 25Cshows the photoresist has been removed and a KOH etch it performed alongthe (111) planes 810 and 812. FIG. 25D shows the resulting beam 814following the KOH etch.

FIG. 26 shows a sensor array configuration where the flexibleinterconnects are metal traces on silicon springs. The silicon device isformed from a single wafer and a mesh of silicon devices andinterconnecting microsprings form a flexible and extensibletwo-dimensional array. Metal interconnects are shown on the siliconsprings. Doped conductive traces are also possible. Although the metalon silicon spring is not as flexible and extensible as the free-standingmetal version of the array, the metal is supported everywhere by siliconso shearing of the metal springs does not pose a concern. Various springgeometries are possible as the springs are fabricated with a plasma etchprocess (i.e. arbitrary spring curvatures can be formed, curved orrectilinear). In the above image, the metal conductive traces appeardark. They are positioned on the silicon which appears light. Thelightest areas are the doped silicon. In this example, the doped silicontraverses a doubly supported beam. Also note that a mote exists aroundthree sides of the stress sensitive area of the device to decouplebending of the springs from beam bending. This helps to reduce artifactof an array bending to conform to a curved surface from beam bending dueto applied loads (the measurand of interest).

FIG. 27 is a view of another metal on silicon spring array. Gold-cappedmetal traces form row-column wires that communicate with a doped tracethat traverses each silicon doubly-supported beam.

FIG. 28 is a view of independent islands of silicon that each carry asensor arranged in a two-dimensional array. A row-column wiring schemeinterconnects the independent silicon devices. The wires arefree-standing metal interconnects or metal interconnects supported bypolymer or other substrates. In this example, the metal interconnectsare supported and formed on silicone rubber. The silicone rubber allowsflexibility and stretchability needed to conform to complex curvatures.A variety of spring geometries can be formed in the metal as their shapeis arbitrarily defined by photolithography.

FIG. 29 is a view of a 30×30 (900 sensor) array. Independent islands ofsilicon each carry a sensor. The sensors are interconnected by metalreleased on silicone-rubber as described above.

This and the following paragraphs generally describe some of theembodiments of the invention, and various alternates. One basicembodiment is an apparatus comprising a sensor element that includes abody having a thickness no greater than 280 μm, a recessed portion and apressure sensitive element that extends over the recessed portion; andan electric circuit operatively connected to the pressure sensitiveelement, where the circuit includes a pressure signal generating circuitelement configured to provide a signal when a force is exerted on thepressure sensitive element. The sensor element can be one of a pluralityof sensor elements. The invention can further comprise a package, wherethe sensor element is located within the package to provide a packagedsensor element, where the sensor element together with the packagecomprises a total thickness no greater than 300 μm. When the inventionincludes a plurality of sensor elements, the plurality can be configuredin an array selected from the group consisting of a one dimensionalarray and a two dimensional array. The sensor element is manufactured bya microfabrication process.

To enable handling, the sensor element can be processed by attaching itwith an adhesive to a handle. The adhesive can be an ethylene vinylacetate polymer. The sensor element can comprise silicon and can besingle crystal silicon. The pressure sensitive element can comprise aconfiguration selected from the group consisting of a diaphragm, acantilever and a doubly supported beam.

For the packaged sensor embodiments, the package can comprise a baselayer, a mid-layer with an opening for the sensor element, and a cappinglayer, where the mid-layer is between the base layer and the cappinglayer. A conductive trace can be located between the base layer and themid-layer and is in electrical contact with the electrical circuit. Insome embodiments, the base layer, the mid-layer and the capping layercomprise the same material, e.g., polyimide. Solder may be locatedbetween the conductive trace and the circuit. The base layer cancomprise polymer, where the mid-layer and the capping layer can eachcomprise at least one adhesive layer and at least one polymer layer. Thebase layer can further comprise a solder shim layer. The solder shimlayer is configured to uniformly support the sensor element between thesensor element and the conductive trace. In other words, the base layercan comprises means for providing a deterministic solder thicknessbetween the sensor element and the conductive trace.

In some embodiments, the conductive trace is in electrical contact with,but is not adhered or bonded to, the electrical circuit. In someembodiments, the conductive trace is not bonded to the electric circuit.In some embodiments, the invention can further comprise means forrelieving residual stress curl of the package. In the diaphragmembodiment, a sealed reference gas volume can be located beneath thediaphragm. Some embodiments include a load amplification element inoperative contact with the pressure sensitive element. The loadamplification element can comprises an electroplated bump.

The electric circuit can comprise implant material implanted into thepressure sensitive element, where the pressure sensitive elementcomprises a substrate selected from the group consisting of an N typesubstrate, a P type substrate, an N type well in a P type substrate anda P type well in an N type substrate. When the substrate comprises an Ntype substrate, the implant material can comprise a P type material,where when the substrate comprises a P type substrate, the implantmaterial can comprise an N type material. Where when the substratecomprises an N type well in a P type substrate the implant can comprisea P type implant and when the substrate comprises a P type well in an Ntype substrate the implant can comprise an N type implant. The implantmaterial is generally activated by annealing. The implant material canbe electrically connected to a conductor that extends off of the sensorelement. The implant material can be electrically connected, at a bondpad, to a conductor that extends off of the sensor element, where thebond pad is stress shielded to prevent damage to the thin-filmconductor.

Embodiments where the pressure sensitive element comprises a diaphragmcan further comprise a thermal compensator comprising at least onepiezoresistive circuit operatively placed on the diaphragm, where thepressure signal circuit element comprises at least one piezoresistivecircuit operatively placed on the diaphragm. Embodiments where thesensor element comprises single crystal silicon, the single crystalsilicon can comprise a <100> crystalline axis and a <110> crystallineaxis, where a thermal compensator can be operatively placed on the <100>crystalline axis and where the pressure signal circuit element can beoperatively placed on the <110> crystalline axis for p-typepiezoresistors in a (100) wafer. The thermal compensator can comprise afirst pair of piezoresistive circuits located about 180 degrees apartand where the pressure signal circuit element can comprise a second pairof piezoresistive circuits located about 180 degrees apart.

In embodiments where the pressure sensitive element comprises adiaphragm, the thermal compensator can comprise at least onepiezoresistive circuit placed on the non-recessed portion, where thepressure signal circuit element comprises at least one piezoresistivecircuit operatively placed on the diaphragm. In some cases, the baselayer, the mid-layer and/or the capping layer comprise a B-stagepolyimide. Sometimes the diaphragm is about 1.5 μm thick, and the bodyhas a thickness that is about 50 μm. A compliant layer can be attachedto the base layer on the side of the base layer opposite to that of themid-layer.

The invention includes a variety of means for connecting the pluralityof sensor elements together. The means for connecting can comprise anextensible interconnect that allows complex curvature conformability.The flexible interconnect can comprise a conductive trace which canfurther be embedded in polymer. The flexible interconnect can comprise ametal trace on a silicon spring. The plurality of sensor elements can bein a row and column electrical configuration.

A wide variety of alternate configurations and methods for are withinthe scope of the present invention. The cantilever or beam can be formedby a deep reactive ion etch followed by a potassium hydroxide etch. Thebasic sensor element can include means for providing thermalcompensation to the signal. The package can comprise a material selectedfrom the group consisting of ceramic, metal, and plastic. The plasticcan be selected from the group consisting of hard plastic and epoxybased plastic. In some case, the package has a thickness no greater than100 μm. The package can comprise a material selected from the groupconsisting of polymide, silicone and mylar. In some cases, the sensorelement comprises a thickness no greater than 100 μm. The pressuresensitive element can comprise a thickness no greater than 50 μm. Insome cases, the sensor element has a thickness no greater than 25 μm.The means for providing thermal compensation can be selected from thegroup consisting of a full wheat-stone bridge, a half wheat-stonebridge, a quarter bridge and a temperature device. The pressuresensitive element can comprise a circuit comprising a circuit elementselected from the group consisting of a piezoresistive circuit element,a piezoelectric circuit element and a capacitive circuit element. Thepresent invention includes the methods for fabricating all of the abovedescribed embodiments.

The foregoing description of the invention has been presented forpurposes of illustration and description and is not intended to beexhaustive or to limit the invention to the precise form disclosed. Manymodifications and variations are possible in light of the aboveteaching. The embodiments disclosed were meant only to explain theprinciples of the invention and its practical application to therebyenable others skilled in the art to best use the invention in variousembodiments and with various modifications suited to the particular usecontemplated. The scope of the invention is to be defined by thefollowing claims.

1. An apparatus, comprising: a sensor element that includes a bodyhaving a thickness no greater than 280 μm, a recessed portion and apressure sensitive element that extends over said recessed portion; andan electric circuit operatively connected to said pressure sensitiveelement, wherein said circuit includes a pressure signal generatingcircuit element configured to provide a signal when a force is exertedon said pressure sensitive element.
 2. The apparatus of claim 1, whereinsaid sensor element is one of a plurality of sensor elements.
 3. Theapparatus of claim 1, further comprising a package, wherein said sensorelement is located within said package to provide a packaged sensorelement, wherein said sensor element together with said packagecomprises a total thickness no greater than 300 μm.
 4. The apparatus ofclaim 2, wherein said plurality of sensor elements is configured in anarray selected from the group consisting of a one dimensional array anda two dimensional array.
 5. The apparatus of claim 1, wherein saidsensor element is manufactured by a microfabrication process.
 6. Theapparatus of claim 1, wherein said sensor element is processed byattaching it with an adhesive to a handle.
 7. The apparatus of claim 6,wherein said adhesive comprises an ethylene vinyl acetate polymer. 8.The apparatus of claim 1, wherein said sensor element comprises silicon.9. The apparatus of claim 1, wherein said sensor element comprisessingle crystal silicon.
 10. The apparatus of claim 1, wherein saidpressure sensitive element comprises a configuration selected from thegroup consisting of a diaphragm, a cantilever and a doubly supportedbeam.
 11. The apparatus of claim 3, wherein said package comprises abase layer, a mid-layer with an opening for said sensor element, and acapping layer, wherein said mid-layer is between said base layer andsaid capping layer.
 12. The apparatus of claim 11, wherein a conductivetrace is located between said base layer and said mid-layer and is inelectrical contact with said electrical circuit.
 13. The apparatus ofclaim 12, wherein said base layer, said mid-layer and said capping layercomprise the same material.
 14. The apparatus of claim 13, wherein saidbase layer, said mid-layer and said capping layer comprise polyimide.15. The apparatus of claim 12, further comprising solder between saidconductive trace and said circuit.
 16. The apparatus of claim 15,wherein said base layer comprises polymer, wherein said mid-layer andsaid capping layer each comprise at least one adhesive layer and atleast one polymer layer.
 17. The apparatus of claim 16, wherein saidbase layer further comprises a solder shim layer.
 18. The apparatus ofclaim 17, wherein said solder shim layer is configured to uniformlysupport said sensor element between said sensor element and saidconductive trace.
 19. The apparatus of claim 11, wherein said base layercomprises means for providing a deterministic solder thickness betweensaid sensor element and said conductive trace.
 20. The apparatus ofclaim 12, wherein said conductive trace is in electrical contact with,but is not adhered or bonded to, said electrical circuit.
 21. Theapparatus of claim 12, wherein said conductive trace is not bonded tosaid electric circuit.
 22. The apparatus of claim 11, further comprisingmeans for relieving residual stress curl of said package.
 23. Theapparatus of claim 10, further comprising a sealed reference gas volumebeneath said diaphragm.
 24. The apparatus of claim 1, further comprisinga load amplification element in operative contact with said pressuresensitive element.
 25. The apparatus of claim 24, wherein said loadamplification element comprises an electro-plated bump.
 26. Theapparatus of claim 1, wherein said electric circuit comprises implantmaterial implanted into said pressure sensitive element, wherein saidpressure sensitive element comprises a substrate selected from the groupconsisting of an N type substrate, a P type substrate, an N type well ina P type substrate and a P type well in an N type substrate.
 27. Theapparatus of claim 26, wherein when said substrate comprises an N typesubstrate, said implant material comprises a P type material, whereinwhen said substrate comprises a P type substrate, said implant materialcomprises an N type material.
 28. The apparatus of claim 26, whereinwhen said substrate comprises an N type well in a P type substrate saidimplant comprises a P type implant and when said substrate comprises a Ptype well in an N type substrate said implant comprises an N typeimplant.
 29. The apparatus of claim 26, wherein said implant materialhas been annealed.
 30. The apparatus of claim 1, wherein said pressuresensitive element comprises a diaphragm, and further comprises a thermalcompensator comprising at least one piezoresistive circuit operativelyplaced on said diaphragm, wherein said pressure signal circuit elementcomprises at least one piezoresistive circuit operatively placed on saiddiaphragm.
 31. The apparatus of claim 1, wherein said sensor elementcomprises single crystal silicon, wherein said single crystal siliconcomprises a <100> crystalline axis and a <110> crystalline axis, whereina thermal compensator is operatively placed on said <100> crystallineaxis and wherein said pressure signal circuit element is operativelyplaced on said <110> crystalline axis for p-type piezoresistors in a(100) wafer.
 32. The apparatus of claim 31, wherein said thermalcompensator comprises a first pair of piezoresistive circuits locatedabout 180 degrees apart and wherein said pressure signal circuit elementcomprises a second pair of piezoresistive circuits located about 180degrees apart.
 33. The apparatus of claim 8, wherein said pressuresensitive element comprises a diaphragm, wherein said thermalcompensator comprises at least one piezoresistive circuit placed on saidnon-recessed portion, wherein said pressure signal circuit elementcomprises at least one piezoresistive circuit operatively placed on saiddiaphragm.
 34. The apparatus of claim 17, wherein said polymer comprisespolyimide.
 35. The apparatus of claim 12, wherein at least one of saidbase layer, said mid-layer or said capping layer comprises a B-stagepolyimide.
 36. The apparatus of claim 10, wherein said diaphragm isabout 15 μm thick, wherein said body has a thickness that is about 50μm.
 37. The apparatus of claim 11, further comprising a compliant layerattached to said base layer on the side of said base layer opposite tothat of said mid-layer.
 38. The apparatus of claim 26, wherein saidimplant material is electrically connected to a conductor that extendsoff of said sensor element.
 39. The apparatus of claim 26, wherein saidimplant material is electrically connected, at a bond pad, to aconductor that extends off of said sensor element, wherein said bond padis stress shielded to prevent damage to said thin-film conductor. 40.The apparatus of claim 2, further comprising means for connecting saidplurality of sensor elements together.
 41. The apparatus of claim 40,wherein said means for connecting comprise an extensible interconnectthat allows complex curvature conformability.
 42. The apparatus of claim41, wherein said flexible interconnect comprises a conductive trace. 43.The apparatus of claim 42, wherein said conductive trace is embedded inpolymer.
 44. The apparatus of claim 41, wherein said flexibleinterconnect comprises a metal trace on a silicon spring.
 45. Theapparatus of claim 1, wherein said plurality of sensor elements areelectrically in a row and column configuration.
 46. The apparatus ofclaim 10, wherein said cantilever or beam is formed by a deep reactiveion etch followed by a potassium hydroxide etch.
 47. The apparatus ofclaim 1, further comprising means for providing thermal compensation tosaid signal.
 48. The apparatus of claim 3, wherein said packagecomprises a material selected from the group consisting of ceramic,metal, and plastic.
 49. The apparatus of claim 48, wherein said plasticis selected from the group consisting of hard plastic and epoxy basedplastic.
 50. The apparatus of claim 3, wherein said package has athickness no greater than 100 μm.
 51. The apparatus of claim 3, whereinsaid package comprises a material selected from the group consisting ofpolymide, silicone and mylar.
 52. The apparatus of claim 1, wherein saidsensor element comprises a thickness no greater than 100 μm.
 53. Theapparatus of claim 1, wherein said pressure sensitive element comprisesa thickness no greater than 50 μm.
 54. The apparatus of claim 1, whereinsaid sensor element has a thickness no greater than 25 μm.
 55. Theapparatus of claim 47, wherein said means for providing thermalcompensation are selected from the group consisting of a fullwheat-stone bridge, a half wheat-stone bridge, a quarter bridge and atemperature device.
 56. The apparatus of claim 1, wherein said pressuresensitive element comprises a circuit comprising a circuit elementselected from the group consisting of a piezoresistive circuit element,a piezoelectric circuit element and a capacitive circuit element.